Koreans did not invent Chinese characters. They actually incorporated the Chinese language into their own language. Hangul is the Korean language, but before that language system was used, Han Ja was used.

So basically, to be literate in Korea (a long long long time ago), you had to have knowledge of classical Chinese characters, which were the basis of Han Ja.

Even today, children in Korea are taught han-ja as it is still used to represent some words.